Induction furnace



Aug. 1, 1950 Filed Nov. 12, 1948 L. A. DREYFUS INDUCTION FURNACE '2 Sheets-Sheet l 1 Illlllllllllll" lIllIllllllllllllllllllllll Illlllll 1950 I L. A. DREYFUS 2,517,098

INDUCTION FURNACE Filed Nov. 12, 1948 2 Shets-Sheet 2 Patented Aug. 1, 1950 INDUCTION FURNACE Ludwig A. Dreyfus, Vasteras, Sweden,

Allmanna Svenska Elektriska Vasteras, Sweden,

assignor to Aktiebolaget,

a Swedish corporation Application November 12, 1948, Serial No. 59,705

' In Sweden November 10, 1947 4 Claims. 1

Electric furnaces in which the heating is effected by means of induced eddy currents must sometimes, for mechanical or metallurgical reasons, possess receptacles of a rather small mean diameter. In such cases it may often be advisable, not only with respect to the insulation and cooling of the inducing windings but also with respect to a saving of conducting material and power losses, to employ a common inducing winding for a plurality of furnace receptacles. The

problem then is to effect a uniform heat production in all the receptacles, this being of the utmost importance for many heat treating processes. A mathematical treatment of the eddy current furnaces shows that the current density of the induced eddy currents and thus the heat production depends not only on the number of ampere-turns per unit of length of the inducing coils but also on the distance between this coil and the material in which the eddy currents are induced, so that this current density decreases as this distance increases. For this reason, it is as a rule not possible to obtain a uniform heat production in a group of receptacles surrounded by a common inducing winding.

Essentially the same problem as in a furnace having several crucibles, which need a uniform heating, may also present itself in other cases, for instance when the furnace contains a plurality of conducting units which mutually form an imperfect conducting connection, so that with respect to the induced currents these units must be regarded essentially as separate units which need a uniform heating. Such a situation may for instance present itself in gas generators or carburetors heated by eddy currents and in which the carbon has the shape of conducting pieces, for instance pieces of coke.

According to the present invention, an electric eddy current furnace having a plurality of conducting units in which eddy current circuits closed in themselves are induced by the action of a common surrounding winding and a winding surrounded by the units, is provided inside the latter winding with an iron core which is separate from the rest of the iron path of the furnace.

In the accompanying drawing, Figs. 1 and 2 show two diagrammatical vertical sections of two formsof electrical eddy current furnaces, said sections being intended to illustrate the flux distribution in furnaces for different arrangements of their inducing windings, while Figs. 3 and 4 show plan views of two forms of the invention and Fig. 5 a diagram of connections for one form of a furnace according to the invention.

In Fig. 1, I designates in a diagrammatical section an inducing winding of an eddy current furnace'and 2 is a laminated iron shell surrounding the winding. The details of the furnace proper are not shown. The fulldrawn lines a designate magnetic lines of flux and the dotted lines D designate level surfaces of the magnetic induction perpendicular to said flux lines a. The inverted value of the distance between the level surfaces forms a direct measure of the force of the magnetic induction in each point. As is clear from the diagram, this force is considerably lower near the central axis of the furnace than near the winding I, especially towards the ends of the furnace.

In the diagram according to Fig. 2, the parts and the curves are designated in the same manner as in Fig. 1 but an internal furnace winding 3 surrounding a laminated iron shell 4 has been added, and it will be seen that the distance between the level surfaces b is practically the same throughout the furnace, and consequently the heat production per surface unit of the parts in which eddy currents are induced will be practically the same. As is apparent from the drawing, shells 2 and I extend beyond the ends of the coils l and 3 respectively.

In Fig. 3, which shows a cylindrical furnace in plan view, the two furnace windings and the external iron shell and the internal iron core are designated in the same manner as in Fig. 2. 5 designates a number of conducting units, of cylindrical shape, which are intended to be heated to essentially the same temperature by induced eddy currents. The units may for instance consist of conducting crucibles having a non-conducting or conducting content, of nonconducting crucibles having a conducting content, or of conducting rods intended to be heated for instance for hardening or annealing. They may also consist of conducting pieces of a more irregular shape, for instance of coke pieces in a gas generator or carburator. In this case it is also particularly important that the head production be uniform in all the conducting pieces so as to keep them at substantially the same temperature.

The furnace according to the invention need not have a circular cross section as shown in Fig. 3 but may have for instance a polygonal or more or less oblong section as shown in Fig. 4 in which, however, only the windings and units to be heated are shown and designated in the same manner as in Fig. 3.

Fig. 5 finally shows an example of the diagram of connections for a furnace according to the invention. The external winding is here still designated by l and the internal winding by 3. These two windings are here connected in series to the terminals of the high frequency generator 6. The main portion of the reactive power necessary for the furnace is delivered by a condenser connected in parallel to the entire furnace winding. For adjusting the distribution of the power between the two winding parts I and 3, it may also be advisable to connect in parallel with either or both of said parallel windings a reactive element which preferably may be adjustable, namely either a condenser I or an inductance 9.

I claim as my invention:

1. An electric eddy current furnace comprising a plurality of crucibles adapted to contain material to be heated, a common inducing winding surrounding said crucibles, a common inducing winding surrounded by said crucibles, a laminated magnetic iron shell surrounding said first winding,'a laminated magnetic iron core separate from said shell and surrounded by said second winding, said shell and core extending beyond the ends of the first and second windings respectively, a source of alternating current, and means connecting said windings to said source.

5 to claim 1 in which diflerent reactive elements are connected in parallel with said two inducing windings taken separately for adjusting the relative currents therein.

4. An electric eddy current furnace according to claim 1 in which a reactive element is connected in parallel with at least one of said two inducing windings.

LUDWIG A. DREYFUS.

REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,078,619 Greene Nov. 18, 1913 1,608,560 Jones Nov. 30, 1926 1,645,526 Gerth Oct. 18, 1927 1,645,704 Ledwinka Oct. 18, 1927 1,744,983 Northrup Jan. 28, 1930 1,771,114 Fry July 22, 1930 1,822,539 Northrup Sept. 8, 1931 1,837,031 Engelhardt et al. Dec. 15, 1931 1,852,215 Northrup Apr. 5, 1932 1,878,532 Northrup Sept. 20, 1932 1,943,802 Northrup Jan. 16, 1934 FOREIGN PATENTS Number Country Date 267,253, Great Britain Mar. 17, 1927 305,620 Great Britain Feb. 8, 1929 492,763 Great Britain Sept. 27, 1938 503,255 Great Britain June 28, 1939 586,822 Germany Oct. 26, 1933 

